The present invention relates to a color temperature selector for selecting a color temperature of a color image displayed on a CRT of a color television receiver.
Recently, the performance and function of a color television receiver have improved greatly. Color television receivers equipped with a color temperature selector have been desired wherein the color temperature of an image can be selected with a remote controller in accordance with user preference or the contents of displayed images.
A conventional color temperature selector generally has a structure such as shown in FIG. 6. This structure will be described while referring to FIG. 6.
As shown in FIG. 6, the emitters of red and blue signal output transistors 1 and 3 each have three resistors connected in parallel with a capacitor. The three resistors have high, middle, and low resistance values, respectively, and are selected by switches.
The base of the red signal output transistor 1 is supplied with a red color difference signal. The emitter thereof is supplied with a luminance signal Y via a red signal drive adjusting variable resistor 13 and via one of resistors 41, 42 and 43 selected by a color temperature selector switch 4 (the resistance values are set such that resistor 41&gt;resistor 42&gt;resistor 43).
Similarly, the base of the blue signal output transistor 3 is supplied with a blue color difference signal. The emitter thereof is supplied with the luminance signal Y via a blue signal drive adjusting variable resistor 15 and via one of resistors 51, 52 and 53 selected by a color temperature selector switch 5 (the resistance values are set such that resistor 51&lt;resistor 52&lt;resistor 53).
The base of a green signal output transistor 2 is supplied with a green color difference signal, and the emitter thereof is supplied with the luminance signal Y via an emitter resistor 14.
The drive level for the green signal is fixedly set.
The color temperature selector switches 4 and 5 operate in unison. The values of the resistors 42 and 52 each having a middle value are set such that when the switches select them, the red and blue signals are driven at a middle level. The drive gain of the output circuit is expressed by the following equation: EQU Drive Gain=(load resistor value Rl)/(emitter resistor value Re)(1)
The smaller the values of the resistors selected by the switches 4 and 5, the larger drive levels are obtained.
When the switches 4 and 5 select the resistor 41 with a high resistance value and the resistor 51 with a low resistance value, the drive level for the red signal becomes smaller than when the resistor 42 was selected, and the drive level for the blue signal becomes larger than when the resistor 52 was selected. Accordingly, a bluish image having a high color temperature is displayed on the screen.
When the switches 4 and 5 select the resistor 43 with a low resistance value and the resistor 53 with a high resistance value, the drive level for the red signal increases and that for the blue signal decreases. Accordingly, a reddish image having a low color temperature is displayed on the screen.
With a conventional color temperature selector as described above, one of the three emitter resistors are exchangeably selected to determine a drive level. In this case, the resistance of each drive transistor takes nearly the same value of the selected emitter resistor when the transistor turns on. The range of values of these three resistors is wide, so that semiconductor switches which are turned on and off by a control signal have been impossible to use.
If the on/off control by a control signal is performed using relay switches or the like, a corresponding number of relay switches are required, resulting in a complicated circuit and high cost which is not practical for use with home electronics such as television receivers.
In view of this, a conventional color temperature selector uses mechanical switches to select a color temperature.
In assembling a conventional color temperature selector using mechanical switches in a television receiver, there are some restrictions such as the mounting space of switches and the connection lead wire length. Furthermore, such a color temperature selector cannot be driven by an electrical control signal, posing a problem that it cannot use a remote controller.